feat: add Veo 3.1 Vertex AI video models

[jynbil1]

Summary

• Adds Vertex AI support for veo-3.1-fast-generate-preview and veo-3.1-generate-preview
• Adds current Vertex AI pricing for Veo 3.1 Fast and Veo 3.1 in the SDK video model registry
• Updates the Next video template model picker, request validation, handler types, and default model
• Adds a focused SDK regression test for Veo 3.1 model IDs and pricing

Fixes #595
/claim #595

Verification

• SDK video model test passed
• SDK type-check passed
• Next video template lint passed
• Next video template build passed
• Prettier check passed for changed files
• Diff whitespace check passed

Notes

• Server package type-check is blocked by a pre-existing src/utils.ts export issue after generating Prisma types.
• Server package lint is blocked before file linting by the package's ESM ESLint config using __dirname.

[vercel]

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